Scientists have long speculated that people have a "set point" for weight that's tough to change. Now researchers think we may have an exercise set point as well, which compensates for extra movement by making us lazy later on.

The Times Well blog reports on a study of 8-10-year-olds at English schools with different PE requirements. The kids at the school with the strictest requirements spent 65% more time exercising during the schoolday than kids at other institutions — but when researchers looked at activity levels across the whole day, they actually evened out. Basically, the kids who moved around a lot at school were lazier when they got home. Or, as study author Terence J. Wilkin says, "activity at one time is met with less activity at another."

Similar studies on animals and adults also imply that we may have a "genetically determined level of preferred energy expenditure" — a certain amount we're predisposed to move around per day, regardless of whether we hit the gym. The implications of this are a little disturbing — are our bodies sabotaging our fitness efforts by slacking off after we exercise? Does making the effort to exercise even matter, if we're going to move around the same amount no matter what?

Most scientists say it still does. Some have questioned Wilkin's conclusions, pointing out that other studies show environment has a profound effect on activity levels. And Wilkin himself says,

Exercise is extremely good for the health of young people, as it is for all of us. It improves metabolic profiles and cardiorespiratory fitness. Our results should not be interpreted to mean that exercise is not worthwhile.

It stands to reason that even if we do have an activity set point, vigorous exercise might be useful — unless we set out to do so, we're probably not just naturally jogging or swimming at some point during the day, and thus we need dedicated exercise time to get our heart rates up. Still, Wilkin's research might explain why people have a hard time losing weight through exercise — their bodies may be working against their efforts.